Controllable semi-active dampers for providing a controllable damper force in real time are described in the commonly assigned U.S. Pat. No. 3,807,678 to Karnopp et al. entitled "System for Controlling the Transmission of Energy Between Spaced Members" which is hereby incorporated by reference herein. The Karnopp et al. '678 patent describes a passive spring 24 arranged in parallel to an active viscous damper 26. The damping characteristics of the damper are actively varied in a real-time fashion as a function of feedback signals such as position, velocity, and acceleration. The system described in Karnopp '678 does not require any user input.
Both Koni and Kayaba Industry Co. Ltd. offer a shock absorber in which the damping force is user adjustable. However, these user adjustable shock absorbers are only passively adjustable in that the valve does not react to changing road conditions in a real time fashion. The valve remains at the position setting that is set by the user. Thus, under certain abrupt changes in road conditions, or abrupt vehicle maneuvers, large undesirable body motions can be experienced, such as during cornering, accelerating or braking.
Other adjustable shock absorbers do react to vehicle roll by sensing such things as lateral acceleration as described in U.S. Pat. No. 5,054,813 to Kakazaki and U.S. Pat. No. 5,087,068 to Fukanaga et al. Inagaki et al., U.S. Patent No. 4,729,459, teaches sensing the force in each shock absorber via piezoelectric elements to determine the occurrence of roll. Dieter U.S. Pat. No. 5,046,008 describes using side-slip angle, yaw velocity, roll angle, and lateral acceleration to determine a condition indicative of roll. In each case, as roll is sensed and compared to a threshold, the shock absorber then adjusts to a hard damping state to deter vehicle roll when said threshold is exceeded. The threshold has to be set tightly to make sure that no appreciable amount of roll occurs before the damper is commanded to a hard state. This has the disadvantage of creating a harsh ride under most non-roll conditions because the null band has to be set so tightly. This harshness is a result of the damper tracking minor road perturbations. Thus, the prior systems experience a tradeoff between providing a smooth ride and controlling roll, neither of which can the prior art systems control optimally.